In 1998 curators at the Courtauld Institute received an anonymous phone call informing them that 11 drawings in their collection were fakes. The caller intimated that he was an associate of the notorious forger Eric Hebborn, who had claimed in his 1991 memoir, Drawn to Trouble, to have sold the institute a fake Rowlandson.

The Courtauld had, in fact, already rumbled the Rowlandson before Hebborn boasted of putting one over on it; now it looked like it could be more than one. The other ten included three sketches by Tiepolo, three by Guardi and a drawing by Michelangelo.

There’s a bit of the bloodhound in every art historian, and when Rachel Hapoienu came in to catalogue drawings in the collection she paid particular attention to this group. Her discoveries prompted the intriguing little show she has co-curated with the Courtauld’s curator of paintings, Karen Serres, an exploration of artistic skulduggery through the histories of some 30 disputed works in the collection, including the 11 on the caller’s list. Some of these have since been authenticated, their existences recorded before Hebborn could hold a pencil; on the Michelangelo the jury is still out.

When the collection was founded by Samuel Courtauld in the 1920s, before the advent of the modern technologies familiar to fans of Fake or Fortune?, spotting fakes was all about connoisseurship – a connoisseurship severely tested by professional forgers expert at covering their tracks. None were more professional than the Sienese, who turned their training as restorers of Renaissance paintings to more profitable use.

Botticelli’s ‘Madonna of the Veil’ (see above) was hailed as a masterpiece when rediscovered by an Italian dealer in 1930 and sold to the Courtauld’s co-founder, Viscount Lee of Fareham, for $20,000. True, the Madonna was a little too pouty for Kenneth Clark who noted her resemblance to a silent movie star, and the wormholes looked drilled, but it wasn’t until the 1990s that EDX analysis confirmed the presence of 19th-century pigments. The picture has since been reattributed to Umberto Giunti, a teacher at Siena’s Institute of Fine Art who trained under the gilder and restorer Icilio Federico Joni, responsible for another forgery in the show. The revival of interest in early Renaissance gold backs around 1900 fostered a flourishing Sienese industry in their production. The punchwork on Joni’s ‘Triptych with Virgin and Child with Saints’ was perfect; he wasn’t to know that X-ray technology would reveal the nails supporting the stucco flames around his gilded frame as machine-made.

Joni spilt the beans in his 1932 Memoirs of a Painter of Old Paintings which, he said, would have been published ten years earlier if dealers hadn’t paid him hush money not to. ‘The artist is not always the baddie,’ says Serres. The show features several examples of sharp practice by dealers: faking the stamps of historic collectors, having drawings reworked in the styles of more famous masters or adding the signatures of more sought-after artists. When Blake became more popular than Romney at the end of the 19th century, a Romney drawing acquired a ‘WB’.

Prolific draughtsmen are easy prey for forgers. A ‘Head of a Man’ signed ‘Corot’ was one of a cache of 2,400 drawings that came to light in the artist’s country studio in 1926, 50 years after his death – all 2,400 by the same hand, not Corot’s. Rodin’s life drawings were liberally faked, though not as easy to fudge as they look. The thigh of the ‘Seated Female Nude’ in the show seems to sprout from her flank, and her pose is far too coy for Rodin; its acquisition was one of Samuel Courtauld’s rare boo-boos.

Collectors are not immune from our human propensity to believe what we want to believe. Looking today at the forgeries of Hans van Meegeren, who made a killing during the second world war flogging fake Vermeers to the Nazi elite, it’s hard to imagine experts being taken in. But Van Meegeren was not only technically inventive, developing a Bakelite recipe to harden his paint surfaces in the oven; he was cunning in his selection of subjects. His copy of Dirck van Baburen’s ‘The Procuress’, given to the Courtauld as a teaching aid, was made when the original, which appears in the background of two paintings by Vermeer and is now in Boston, was lost. With the help of a copy in the Rijksmuseum, he ‘found’ it. When the monetary value of an artist’s work is disconnected from its aesthetic worth, the temptation to fraud becomes irresistible.

If sold today, Serres estimates that Viscount Lee’s ‘Botticelli’ might fetch £10,000, as against the £67 million paid for a Botticelli portrait in 2021. Yet it remains a beautiful painting. Among the fascinating questions raised by this show is the one famously posed by Van Meegeren at his trial: how a painting that was worth millions yesterday can be worth nothing today, when the picture has not changed. ‘What has?’